Increased opsonization of a prtH-defective mutant of Porphyromonas gingivalis W83 is caused by reduced degradation of complement-derived opsonins.

Periodontitis is a disease of the supporting structures of the teeth that is caused by bacteria whose common ecologic niche is the gingival crevice or the periodontal pocket. Tissue destruction occurs in spite of both local and systemic immune responses against such bacteria. Porphyromonas gingivalis is considered to be an important pathogen in some forms of human periodontitis and is particularly interesting because of its multiplicity of virulence factors. We have previously observed that phagocytosis-resistant invasive strains of P. gingivalis proteolytically degrade C3 and IgG and accumulate less C3-derived opsonins during complement activation. We recently have cloned the prtH gene from P. gingivalis W83 that encodes a 97-kDa active protease, which has the capacity to degrade purified C3 protein. By using this cloned gene we created an allelic exchange mutant of P. gingivalis W83, designated V2296, in which the prtH gene was inactivated. This mutant was previously shown to be less virulent than its parent strain W83 in a mouse model of bacterial invasiveness. In the present study we have assessed the relative capacity of V2296 and W83 to be opsonized by complement and to be taken up by PMNs. The data demonstrate that V2296, in comparison with its parent strain W83, is less able to degrade C3 and that it accumulates significantly greater numbers of molecules of C3-derived opsonins on the bacterial surface in the form of C3b and iC3b during complement activation. Furthermore, opsonized V2296 is taken up in much higher numbers by human PMNs than W83, suggesting that the prtH gene product may be important in evasion of host defense mechanisms.